Expandable films and molded products therefrom

ABSTRACT

A thin, tacky, non-pourable film of incompatible in situ-expandable thermoplastic particles and thermosettable matrix resin that contains an essentially uniform density and thickness across the breadth of the film. The in situ-expandable mass is not pourable yet can be easily dispensed in a uniform manner within a mold and thereafter expanded to the dimensions of the mold. Composites and reinforced compositions, as well as methods of molding, are disclosed.

This application is a division of application Ser. No. 07/693,695, filedon Apr. 30, 1991, now U.S. Pat. No. 5,234,757.

BRIEF DESCRIPTION OF THE INVENTION

Thin, tacky film of in situ-expandable thermoplastic particles in athermosettable matrix resin that contains an essentially uniform densityand thickness across the breadth of the film. Composites of the filmsand molded syntactic foam products are made by thermally curing thematrix resin in the films.

BACKGROUND TO THE INVENTION

SynCore®, sold by The Dexter Corporation, Adhesive & StructuralMaterials Division, Pittsburgh, Calif. 94565 U.S.A., is a syntactic foamfilm that takes the place of more expensive prepreg plies in stiffeningcritical structures. This isotropic foam is a composite materialconsisting of preformed microballoons in a thermosetting matrix resin. Awide variety of preformed microballoons and matrices can be combined tomake SynCore® materials. Glass is the most common microballoon materialof construction, but quartz, phenolic, carbon, thermoplastic and metalcoated preformed microballoons have been used. Epoxies curing at 350° F.(177° C.) and 250° F. (121° C.) are the most common thermosetting matrixresins, but matrices of bismaleimide (BMI), phenolic, polyester, PMR-15polyimide and acetylene-terminated resins have been used to produceSynCore® syntactic foams. As a result of the variety of materials thatsuccessfully make SynCore®, they are tailorable to a variety ofapplications. There is a version of SynCore® available that will co-curewith all known available heat-cured composite laminating resins.Syncore® allows sandwich core concepts to be used in a thinner dimensionthan previously possible. The thickness limit on honeycomb cores isapproximately 0.125 inch. Syncore® is available in 0.007 to 0.125 inch(0.18 mm to 3.2 mm) thicknesses but can be made in thinner or thickersheet forran. Other core materials such as wood and sheet foam can bemade thin, but are not drapable and generally require an expensive/heavyadhesive film to bond to the partner composite components. In addition,Syncore® possess excellent uniformity in thickness which provides theability to assure quality for the composite in which it is used as acomponent. Syncore® is typically used to replace prepreg plies where theintent is to increase stiffness by increasing thickness.

Designing with Syncore® is straightforward because all of the analysismethods that apply to other core materials such as honeycomb apply toit. Flexural stiffness of fiat plates and beama increases as a cubicfunction of thickness allowing a lighter, stiffer lamination than couldbe made from prepreg plies alone. Since Syncore®, on a per volume basis,typically costs less than half of a comparable carbon prepreg, it alsoleads to a lower cost lamination. This is illustrated by the following:

1) Adding one ply of 0.020 inch Syncore® and eliminating one ply ofprepreg does not change the weight or cost significantly, but nearlydoubles the flexural rigidity.

2) Adding one ply of 0.020 inch Syncore® and eliminating three plies ofprepreg sharply decreases the cost and weight with a small decrease inrigidity.

3) Adding one ply of 0.040 inch Syncore® and eliminating three plies ofprepreg provides lower weight, cost and sharply increases rigidity.

4) The introduction of unidirectional tape allows a further increase inperformance at lower cost and weight at nearly the same thickness.

5) A hybrid tape/fabric/Syncore® construction gives a very attractiveset of weight and cost savings coupled with a 3.4 times increase inflexural rigidity.

Syncore® has been recommended for thin composite structures in anyapplication where flexural stiffness, buckling, or minimum gaugeconstruction is used. It has been shown to save weight and material costin carbon fiber composites. It has been offered to save weight atapproximately the same cost in the case of glass fiber composites.Illustrative applications are covered in U.S. Pat. Nos. 4,861,649,patented Aug. 28, 1989, and U.S. Pat. No. 4,968,545, patented Nov. 6,1990.

The manufacturing methods for employing Syncore® are very similar tothose used for prepregs. Because it is not cured, it is tacky and verydrapable when warmed to room temperature and is easier to lay-up than acomparable preprag ply. It can be supplied in supported forms with alight weight scrim to prevent handling damage when it is frozen. Itrequires cold storage like prepregs, usually 0° F. (-17.7° C.) or below.The various Syncores® materials typically have a room temperatureout-time that is much longer than their companion prepregs. Syncore® isless sensitive to cure cycle variations than prepreg making thecontrolling factor the composite cure cycle selection. It will cure voidfree under full vacuum or low (e.g. about 10 p.s.i.) autoclave pressure.It has been cured at up to about 150 p.s.i. without exhibiting ballooncrushing.

In a typical application, a sandwich of Syncore® and prepreg, such as athicker layer of Syncore® between two thinner layers of prepreg, areheld together under heat and pressure to cure the structure into astrong panel. Typical sandwich constructions of this nature are shown inU.S. Pat. Nos. 4,013,810, 4,433,068 and 3,996,654. Such compositestructures typically are produced in fiat sheets and in separatablemolds to obtain various desired shapes.

Though Syncore® will cure void free under significant reduced pressureor when put under pressure, it would be desirable to avoid those costlyconditions to achieve void reduction. It would be desirable to have amaterial that has the properties of Syncore® but achieves void freeconstruction without costly full vacuum operations or low autoclavepressure systems. These methods are typically batch type operations thatmaterially add to the cost of making the composite.

There are certain applications in which it is desirable to have theproperties of a uniform thin drapable syntactic foam film in processingthe formation of a laminated composite, yet have the capacity toautogenously expand so as to fill any void space existing in thecomposite's structure so as to minimize the effects of macro and microvoid defects at interlaminate interfaces.

These interlaminar interfacial micro or macro void spaces are magnifiedby the irregularity of the reinforcing layer of the composite structure.For example, if the composite is of a layer of prepreg-derived carbonfiber reinforced thermosetting resin material, bonded to a syntacticfoam, such as a Syncores® thin uniform film, the layer containing theprepreg-derived material will have an irregularly shaped surface and theSyncore® layer will have a relatively smooth uniform surface. Though theSyncore® is tacky and drapable, it is incapable of filling in all of theirregularities of the prepreg-derived layer. Application of a fullvacuum or the use of a low pressure autoclave can be used tosignificantly reduce the void space, but complete avoidance of microvoids is not readily achievable. Also, conforming Syncore® to theirregular surface causes transfer of the irregularity to the oppositesurface of the Syncore® film. Such surface irregularity transfer may beavoided by sandwiching the Syncore® film using heat and pressure, suchrepositions the film's matrix resin and the microspheres so that thefilm within the sandwiched structure loses its original uniformity.

It would be desirable to be able to adequately bond a syntactic foamthin film to an irregular surface¹ and fill the defects in the surfacewithout tranferring the shape of the defects to the unbonded side of thefilm. It would also be desirable to be able to adequately bond asyntactic foam thin film to a surface and, without the use of vacuum orlow pressure autoclaves, fill the micro voids with the syntactic foamwithout repositioning the film's matrix resin and microspheres.

An advantage of Syncore® for many applications resides in its uniformityof distribution of the microsphere throughout the matrix resin. Suchmicrospheres remain essentially intact throughout the cure cycle. As aresult, it is not possible to have the microspheres concentrate at oneor more surfaces, or one or more other locations in the final composite.It would be desirable to have a drapable thin film, having the handlingqualities of Syncores®, but which would allow the production of asyntactic foam having a controllable density gradient that accommodatesspecific end use applications.

There are a number of applications in which a thin film syntactic foamcould serve as a seal to preclude the passage of gases and liquids. Insome applications, the seal could be subjected to abrasion forces. Itwould be desirable to have a thin film syntactic foam that can beapplied in a manner that allows it to be a sealant to gas or liquid flowin a confined space and be able to withstand abrasive forces.

There is a body of technology directed to fabricating expandablethermoplastic resinous material. For example, U.S. Pat. No. 2,958,905,patented Nov. 8, 1960, is directed to a method of making foam structuresfrom particulate expandable granular thermoplastic resinous materialcontaining in the particles a blowing agent for further expansion of theparticles. A considerable number of thermoplastic resins are describedas suitable for this purpose. The blowing agents are the conventionalones recommended for that application. The expandable granularthermoplastic resinous material may be admixed with a thermosettingresin to generate on curing the exotherm needed to expand the expandablegranular thermoplastic resinous material. The resulting mass can bepoured into a mold to make a number of products. The patentees indicatesthat the expandable granular thermoplastic resinous material can beformed in the presence of non-expandable filler materials such as staplefibers from a variety of sources, and the mixture fed to a mold forforming an expanded product. The resulting foamed product may bedesigned to adhesively bond to a fabric layer for reinforcement of thefoamed product. The density of the foamed product can be controlled bythe amount of the expandable material fed to the mold. According to thepatentees, starting at column 12, lines 5 et seq., molded products areformed by charging the mold "with the expandable material in any desiredmanner including manual filling or pneumatic conveyance thereof."According to the description at column 12 relating to FIGS. 3 and 4 (seecolumn 12, lines 16-32):

"a considerable occurrence of void and hollow spaces occurs between thecharged expandable beads 21 in the mass to be fabricated, each of which(in the case of preexpanded material) is a foam structure containing aplurality of internal cells or open spaces. When the liquid exothermussubstance is added between such interparticle voids, the heat from itsspontaneous self reaction causes the beads to expand whereby, asillustrated in FIG. 4, the expanded and fabricated particles 22 forceout a substantial portion (and frequently most) of the exothermussubstance excepting for a minor quantity of reacted material 23 whichremains, frequently as an interlaced and interlinking network betweenthe expanded particles to assist in holding the expanded, cellular foamparticles together." (Emphasis added)

U.S. Pat. No. 2,959,508, patented Nov. 8, 1960, describes anothervariation of using expandable thermoplastic particles. In this patent,the unexpanded particles and the exothermus substance, such as an epoxyresin, are first mixed and then poured into the mold to form a compositefoam of the two when the exothermus substance heats up the mixture andcauses the blowing agent to volatilize.

Thermosetting resins have had blowing agents incorporated in them (seeU.S. Pat. No. 3,322,700, patented May 30, 1967) to form expanded moldedproducts and recently, such types of resin systems have includedpreformed microspheres in the formation of partial syntactic foam films.These expanded thermosets comprise a more open cellular structure unlikethat of syntactic foams, and the inclusion of preformed microspheresdoes not alter that condition.

There are commercial molding processes that utilize tacky sheets ofthermosetting resins and reinforcing material. One such process involvesthe compression molding of sheet molding compounds ("SMC"). In thatprocess, a thermosetting polyester resin filled with staple glass fiberand low profile thermoplastics, are sheeted out and thickened into apourable paste retained between release surfaces such as polyethylenefilm. Chunks of the thickened paste are casually deposited around thesurface of the mold by hand, and on closing the mold with heating, thepaste is liquified and it, and its fiber loading, are redistributedaround the mold to fill it up and form the desired molded article. Inother word, the chunks of sheets of SMC represent a convenient way inwhich to add a liquifiable moldable material to the mold. This processis presently commercially practiced in a number of industries.Advantages of the process are the convenience of storing moldablemixture and the ease of loading a mold with the molding composition.

THE INVENTION

This invention relates to a thin, tacky film of incompatible insitu-expandable thermoplastic particles in a thermosettable matrix resinthat contains an essentially uniform density and thickness across thebreadth of the film. The invention is directed to a moldable insitu-expandable mass that is not pourable yet can be easily dispensed ina uniform manner within a mold and thereafter expanded withoutliquification to the dimensions of the mold. The in situ-expansion canbe carried out without major redistribution of the mass to form asyntactic foam thermoset (cured) article with a predetermined densitypattern.

This invention is directed to moldable, in situ-expandable, filmscomprising a mass of in situ-expandable thermoplastic particles ofdifferent expandabilities, uniformly distributed in a matrixthermosetting resin that is incompatible with the thermoplastic polymerof the in situ-expandable particles. This incompatibility existsthroughout the thermal in situ-expansion cycle in forming the thermosetsyntactic foam molded structure. During this cycle, the incompatibleexpandable thermoplastic particles sufficiently softens while at thesame time expansion agents therein volatilize so as to reform theparticles into hollow microsphere whose outer walls comprise thethermoplastic polymer, forming closed microcells. A significantadvantage of the invention is the formation of thermoset syntactic foamfilms of uniformly distributed expanded closed-cell microspheres thathave a density less than thermoset Syncore® containing preformedmicrospheres (not in situ-formed), yet possesses comparable propertiesfor replacing more expensive prepreg plies in stiffening criticalstructures.

This invention relates to a thin, uniform, tacky, non-pourable film ofan incompatible mixture of in situ-expandable thermoplastic particlesdispersed in a thermosettable matrix resin. The film can be accuratelydispensed in a mold without pouring and, upon subjecting the dispensedfilms to heat, obtaining a cured syntactic foam. The invention alsoincludes the ability to heat the dispensed film in the mold in a unevenmanner to effect a thermal gradient in the mold and obtain a curedproduct having a density gradient throughout that is responsive to suchthermal gradient. As a result of the invention, there may be obtainedmolded syntactic foam structures possessing stiffness and strengthvariability or uniformity depending on the end use application.

This invention is directed to a tacky and drapable, nonpourable filmhaving a uniform (±10%, preferably ±5%) thickness throughout, betweenabout 1.5 millimeters to about 3.5 millimeters, that contains (i) acontinuous phase of a thermosetting matrix resin system and (ii) adiscontinuous phase of particles of a in situ-expandable thermoplasticpolymer containing an expansion agent therein. Both (i) and (ii) areuniformly distributed throughout the film, so that upon expansion of thethermoplastic polymer into microcells in the film, the resulting film isa thermoset thin film syntactic foam the thickness of which is about1.01 to about 4 times greater, preferably about 1.1 to about 3.5 timesgreater, than the non-expanded film. A feature of the non-pourable,tacky and drapable films is that while their thicknesses are uniform,the resultant cured syntactic foam may vary considerably in terms ofdensity and thickness because of molding conditions. It is desirablethat the thin, non-pourable, drapable non-expanded uniform film becapable, upon expansion by uniform application of heat throughout thefilm, while free of any confinement, form an expanded film of uniform(±10%, preferably ±5%) thickness throughout.

A feature of the films of the invention is that the thermoplasticparticle softens sufficiently for expansion at the temperature underwhich the thermosetting matrix resin undergoes cure. Since suchthermosetting resins are curable to temperatures as high as 400° C.,essentially all thermoplastic polymers are amenable for use as thethermoplastic particle component.

The invention allows the facile production of thermoset syntactic foamsof unique conformance to and predetermined density within the moldedvolume of confinement. The "molded volume of confinement" means thatmold space occupied by thermoset syntactic foams of the invention andphysically encompassing the boundaries of the thermoset syntactic foam.The molded volume of confinement is restricted by metal mold surfacesencompassing the mold volume within which expansion of the nonpourable,tacky and drapable film occurs. The mold volume of confinement is alsorestricted by other materials to which the thermoset syntactic foam ofthe invention adheres in the molding operation, to form a compositestructure. The other materials may be derived from thin metal films orfoils (such as aluminum, steel, titanium, and the like), fabrics,prepregs, composites derived from molding prepregs, other fiberreinforced composites, preformed but uncured syntactic foams of othercomposition, and the like. In the typical case, expansion of the thinfilms of the invention fill the molded volume of confinement whereas theother materials occupy no more of the mold's interior before cure asthey do after cure.

The invention encompasses a process as well as products. The processcomprises defining a molded volume of confinement (the mold with orwithout other materials therein) and a thermoset syntactic foam densityfor the resulting molded thermoset syntactic foam. Then at least onelayer of an amount of the non-pourable, tacky and drapable film having auniform thickness, between about 1.5 millimeters to about 3.5millimeters, that contains (i) a continuous phase of a thermosettingmatrix resin system and (ii) a discontinuous phase of particles of a insitu-expandable thermoplastic polymer containing an expansion agenttherein, is deposited and distributed in the mold to achieve the defineddensity. Both (i) and (ii) are uniformly distributed throughout thefilm, so that upon expansion of the thermoplastic polymer intomicrocells in the film, the resulting film is a thermoset thin filmsyntactic foam the thickness of which is about 1.01 to about 4 timesgreater, preferably about 1.1 to about 3.5 times greater, than thenon-expanded film. It is possible to predetermine the uniformity ofexpansion and the resulting density of the thermoset syntactic foambecause both (i) and (ii) are uniformly distributed throughout the film,so that upon expansion of the thermoplastic polymer into microcells, theresulting film is a thin film syntactic foam the thickness of which, asnoted above, is greater than the non-expanded film. When sufficientenergy is applied to the mold to advance the cure of the thermosettingmatrix resin in the tacky and drapable film, to a temperature so as tosufficiently soften the thermoplastic particles and volatile theexpansion agent therein, a syntactic thermoset foam is produced. Themold cycle is completed when the desired density is achieved. Then theresulting molded product encompassing the syntactic foam is withdrawnfrom the mold.

As noted above, the thin and drapable in situ-expandable tacky films maybe composited with other materials. A simple and practical composite maybe one which strengthens the film prior to expansion and conversion tothe thermoset state. The non-pourable, thin, drapable tacky films can behandled without supporting material. However, to avoid premature curingof the film and to assist its handling, it may be cold stored likeprepregs, usually at 0° F. (-17.7° C.) or below, and kept in thatcondition prior to use. Similar to the various Syncore® materials, thefilms of the invention typically have a room temperature out-time thatis much longer than the companion prepregs with which they would bemolded. Because it is not cured, the film of the invention is tacky andvery drapable when warmed to room temperature and is easier to lay-upthan a comparable prepreg ply. It is desirable to make the film insupported forms with a lightweight scrim to prevent handling damage whenit is frozen. In general, it will be desirable to affix the films toother, more durable thin layers that take handling better. For example,the non-pourable, thin, drapable, in situ-expandable films may becalendared to other layers, such as, to scrims, foils and plastic films.One convenient method of affixing handling materials to the films is tosandwich it between plastic films. Adhesion of the film to the handlingmaterial typically relies on its tackiness. If the handling material isan open scrim, such as a woven, nonwoven or knitted scrim, thedrapability of the film assists bonding because the film sags about theindividual fibers or filaments of the fabric, and will interbond throughthe opening in the scrim.

In addition, the films of the invention my be composited with aconventional syntactic foam that comprises thin films of uniformthickness which contain rigid preformed microballoons uniformlydispersed in a resin matrix. The syntactic foam composited with the insitu-expandable film may be any of the SynCore® syntactic foams withwhich it would co-cure.

In another embodiment of the invention, the thin and drapable tackyfilms of the invention may be composited with a layer of a prepreg andthe composite deposited in a mold for forming a product of theinvention. In that case, the prepreg/thin and drapable tacky filmcomposite can be laid up in a mold in a variety of configurations toform a lightweight thermoset composite having good strength andstiffness.

In a further embodiment of the invention, the thin and drapable tackyfilm of the invention may be composited with more than one layer ofmaterial. In particular, the composite may comprise a layer of thehandling material and at least one layer of another material such as apreformed syntactic foam layer or a prepreg layer, or a combination ofthe two. Alternatively, the composite may comprise at least two layersof handling material, such as a layer of scrim and a layer of foil ortwo layer of foil or one layer of scrim and two layers of foil, or onelayer of scrim and one layer of plastic, and the like.

A highly preferred embodiment of the invention comprises a thin anddrapable tacky film comprising a mass of staple thermoplastic fibers andexpandable thermoplastic non-fibrous particles uniformly distributed ina non-pourable matrix containing a thermosetting resin that isincompatible with the thermoplastic polymer of the fibers and theexpandable particles. The thermoplastic fibers have a T_(m) or T_(g)that is greater than the cure temperature of the matrix resin. Thethermoplastic polymer of the expandable particles soften at atemperatured which is less than the cure temperature of the matrixresin. This fiber reinforced film preferably contains the fibersoriented in a direction primarily essentially parallel to the film'ssurface. When the film is expanded into a syntactic foam, the fibers maybe caused to concentrate at the surface to form a tough, abrasionresistent surface. This embodiment, in general, forms molded syntacticfoams that possess unique surface abrasion resistence when compared withother syntactic foams. This embodiment provides syntactic foams suitablefor aerospace applications, where the material needs to be tough towithstand erosion forces or impaction yet be lightweight, a universalrequirement in aircraft applications.

An interesting embodiment of the invention involves scrolling thenon-pourable, thin, drapable film, preferably while adhered to a scrimlayer, into small diameter tubes about which are adhered prepreg layerscontaining carbon fiber reinforcement to form a composite tubecontaining a small hole in the center. Such composite tubes, when viewedcross-sectionally, have a donut appearance. When the tube is cured, thehole in the interior may be fully or partially filled with the expandedsyntactic foam to provide a stiff inner core, along the lines describedin U.S. Pat. No. 4,968,545, patented Nov. 6, 1990.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective (partially isometric) illustration ofa calendaring system by which to make the in situ-expandable thin filmsof the invention.

FIG. 2 is a schematic perspective (partially isometric) illustration ofa variation of the calendaring system of FIG. 1 which has means forincluding a supporting scrim layer in the thin film.

FIG. 3 is a schematic perspective (partially isometric) illustration ofa variation of the calendaring system of FIG. 2 which has means forsandwiching a supporting scrim layer in the thin film.

FIG. 4 is a schematic perspective (partially isometric) illustration ofa variation of the calendaring system of FIG. 1 which has means forincluding in the calendaring step a supporting scrim layer within thethin film.

FIG. 5 is a schematic cross-sectional edge-view of the feed end of thecalendaring operation of FIG. 4, characterizing the particles of insitu-expandable thermoplastic polymer uniformly mixed in a thermosettingresin matrix.

FIG. 6 is a schematic cross-sectional edge-view of the calendaringoperation of FIG. 5, characterizing uniformly distributed staple fibersamongst the particles of in situ-expandable thermoplastic polymerdistributed uniformly in the thermosetting resin matrix.

FIG. 7 is an schematic edge-view of a thin film segment formed in thecalendaring operation of FIG. 6 illustrating the manner of orientationof the staple fibers in the thin film of matrix resin.

FIG. 8 in an perspective end-view of a mold containing the thin film ofthe invention, suitable for forming molded pieces.

FIG. 9 is a plan end-view of the mold of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

As note previously, syntactic foam films, such as Syncore®, aretypically used in stiffness critical applications. The bending stiffnessof a structure is proportional to the third power of thickness (T³). Arelatively slight fluctuation in thickness will result in asignificantly large variation in stiffness. That art teaches us thatcontrolling the film thickness is a crucial manufacturing parameter inmaking a molded composite structure from syntactic foams.

Space volume (foam cells) in thin uniform syntactic foam films such asSyncore® is achieved by substituting light hollow microspheres for therelatively dense matrix resin on a volume to volume basis. The amount ofspace volume that can be achieved is limited by a physical barrier and aprocess obstacle.

The physical barrier occurs at maximum packing fraction. Recognizingthat the objective involves packing spheres into a fixed volume, maximumpacking occurs when point-to-point contacts are developed betweenadjacent/nearest packing spheres. Beyond this point, it is physicallyimpossible to pack any additional spheres into this fixed volume. Formonodispersed spheres, this represents about 64% of the packing volume.In commercially available multi-dispersed sphere systems, the weightreduction is limited by the particular system's packing fraction.

From a processing standpoint, adding glass microspheres to a matrixresin increases resin viscosity, similar to that of adding fillers orthixotropic agents to the resin. This viscosity increase is due to aninteraction between the flowing resin and the surface of the sphere. Insimplistic terms, as the resin moves past a sphere, it imparts an equaland opposite drag force on the surface of the sphere and develops ashear stress through the center of the sphere. Typically, the shearstrength of hollow spheres is low and during processing the resin'sviscosity increases proportionally to the volume of hollow spheresadded. As a result of the proportional increase in shear stress causedby the resin's increased viscosity, sphere damage/fracture occurs. Thisshear stress is the primary cause of sphere damage/fracture duringprocessing. Once the spheres are damaged, the weight saving advantagefrom the use of hollow spheres is negated.

This invention establishes that these sphere related limitations areavoided by the use of expandable thermoplastic particles to in situgenerate microspheres from a thin film to produce a thin (albeitthicker) syntactic foam film. These in situ-expandable thermoplasticparticles possess behavior and processing characteristics similar topigments and fillers. The average particle size of the thermoplastic insitu-expandable particles are typically an order of magnitude, or more,smaller than the pre-expanded hollow sphere used in commercial thin filmsyntactic foams. For example, for a given volume, a significantlygreater number of unexpanded particles may be added to a given volume ofresin compared to the pre-expanded spheres. Since expansion takes placein situ during the cure step, the shear sensitivity of pre-expandedhollow spheres does not become a problem.

The packing of the expanded spheres is also more efficient because of insitu-expansion. During cure, the matrix resin's viscosity, even thoughnon-pourable at normal handling temperatures, such as at about roomtemperature (about 15°-37° C.) or conventional handling temperatures(which can vary from application to application), decreases withincreasing temperature and since the unexpanded spheres are not in apoint-to-point contact configuration, their environment is mobile andthis allows each particle to expand within the film. This mobilityresults in a more densely microsphere-packed lattice. However, in thegeneral practice of the invention, the film will be cut to the size ofthe mold. As a result, because the expansion forces arethree-dimensionally directed, this mobility is initially primarily inthe upwardly z direction rather than in the laterally xy direction. Asexpansion takes place, the film's edge surfaces enlarge, so they exposemore particles to heat through the edge surfaces. As a result, more ofthe particles at the edge surfaces initiate expansion so that a greaterproportion of the particles continue to expand in the z direction.##STR1##

The upward expansion is further magnified by having the film placed inthe mold such that the edges of the film abut or essentially abut thewall or walls of the mold. The free expansion of the particles intomicrospheres is inhibited by the confining walls so that the internalexpansion forces in the particles at the walls are directed more in thez direction initially. One might expect that the edges of the film wouldrise to greater heights in an otherwise vertical free expansion, butthis is not the typical case. Free vertical expansion of the filmresults in a generally uniform rise of the film. This means that thesurfaces of the expanded film which eventually contact the walls of moldare essentially uniform in thickness resulting in a molded product ofexceptional uniformity both in density but also in surface skinthickness at the edges and surfaces.

The film of the invention can be made in a number of ways and with avariety of resin systems to achieve the advantages herein set forth. Theformulation of the film will be dictated by the specific end-useapplication of the film and the resultant molded syntactic foam, as wellas the method employed in making the film. Therefore, it is not intendedthat this description should be limited to any specific application andto any specific formulation and process of manufacture.

The thermosetting matrix resin suitable for use in the inventioncomprise those typically used in the manufacture of Syncore® syntacticfoam. For example, epoxies curing at 350° F. (177° C.) and 250° F. (121°C.) are the most common matrix resins, but matrices of bismaleimide(BMI), phenolic, polyester, PMR-15 polyimide and acetylene terminatedresins that have been used to produce SynCore® products, are usable inthe practice of the invention. However, the invention includes as well,other thermosetting resins; indeed, the invention includes the family ofthermosetting resins. For example, thermosetting resins from acrylics,polyurethanes, free-radically induced thermosetting resin, and the like,may also be used in the practice of the invention. As a restfit of suchconsiderable choices in thermosetting resins, the thin insitu-expandable films of the invention are tailorable to makingsyntactic foams for a wide variety of applications.

Preferably, the invention embraces the use of thermosetting resins thatfind use in adhesive applications thereby providing the desiredtackiness to the film. Such allows the thin film to be appliedconveniently to any substrate and by virtue of the drapability of thefilm, have the film cling to the substrate throughout processing andcure, and configure to the substrate.

As noted, the thin film is non-pourable and tacky. This condition can beachieved in a number of ways. Many thermosetting resins are solids atabout 23° C. and many of them are liquids at this temperature. Bothkinds of resins can be made non-pourable and tacky. For example, a resinwhich is solid and a resin which is liquid can be combined to form amixed resin system that is non-pourable and tacky. In addition, a solidor liquid thermosetting resin can have incorporated in it a variety ofdiverse materials which will render the resin non-pourable atconventional handling temperature conditions and non-pourable and tackyat room temperature (about 15°-37° C.). Conventional handlingtemperatures are defined as a temperature of between about -20° C. toabout 43° C.²

Though the in situ-expandable thermoplastic particles will render aliquid thermosetting resin more viscous, they alone are not effectivefor making the film non-pourable. If the thermosetting resin is solid,it can be calendared into a film by melting the resin with heat underconditions that avoid condensation or addition of the resin to athermoset condition (C-stage). If the resin is a liquid, it can beblended with thixotropic agents, other solid resins and/or liquid orthermoplastic elastomeric modifiers to convert the resin from a liquidto a non-pourable and tacky material.

The typical thermosetting resin is an A-stage resin. In some cases, itmay be desirable to utilize a B-stage resin but in the typical case,such is done in combination with an A-stage resin. Such B-stage resinwill affect the viscosity of the resin formulation but they are notrelied on to achieve the level of non-pourability for the most effectiveoperation of the invention.

A preferred class of thermosetting resin in the practice of theinvention are the epoxy resins. They are frequently based, inter alia,on one or more of diglycidyl ethers of bisphenol A(2,2-bis(4-hydroxyphenyl)propane) or sym-tris(4-hydroxyphenyl)propane,tris(4-hydroxyphenyl)methane, their polyepoxide condensation products,cycloaliphatic epoxides, epoxy-modified novolacs (phenol-formaldehyderesins) and the epoxides derived from the reaction of epichlorohydrinwith analine, o-, m- or p-aminophenol, and methylene dianaline.

The epoxy resins suitable in the practice of the invention include thevarious established thermosetting epoxy resins conventionally employedin making prepregs, especially carbon and graphite fiber reinforcedprepregs. It is desirable that the epoxy resin be a low or lowerviscosity version to facilitate film formation. Illustrations ofsuitable epoxy resins include, e.g., one or more of diglycidyl ethers ofbisphenol A (2,2-bis(4-hydroxyphenyl)propane), such a those of thefollowing formula: ##STR2## or sym-tris(4-hydroxyphenyl)propane ortris(4-hydroxyphenyl)methane, their polyepoxide condensation products,cycloaliphatic epoxides, epoxy-modified novolacs (phenol-formaldehyderesins) of the formula: ##STR3## wherein n is 0-1.8, preferably 0.1-0.5.

Other epoxy resins may be combined with the above epoxy resins or usedalone. They include, e.g., 3,4-epoxy cyclohexyl methyl-3,4-epoxycyclohexane carboxylate, vinyl cyclohexene dioxide, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy) cyclohexane-meta-dioxane, bis (3,4-epoxycyclohexyl) adipate, and the like.

The epoxy resins of the invention are combined with hardeners which curethe resin to a thermoset condition. The preferred hardeners are aminecompounds, ranging from dicyandiamide, to ureas, to aliphatic andaromatic amines. A preferred class of hardeners are the aromatic aminesencompassed by the formula: ##STR4## Q is one or more of a divalentgroup such as --SO₂ --, --O--, --RR'C--, --NH--, --CO--, --CONH--,--OCONH--, and the like, R and R' may each independently be one or moreof hydrogen, phenyl, alkyl of 1 to about 4 carbon atom, alkenyl of 2 toabout 4 carbon atoms, fluorine, cycloalkyl of 3 to about 8 carbon atoms,and the like, x may be 0 or 1, y may be 0 or 1 and is 1 when x is 1, andz may be 0 or a positive integer, typically not greater than about 5.

Preferred hardeners are diamines of the formula: ##STR5##

Another preferred class of hardeners are the aliphatic amines such asthe alkyleneamines. Illustrative of suitable alkyleneamines are thefollowing:

monoethanolamine

ethylenediamine

N-(2-aminoethyl)ethanolamine

Diethylenetriamine

Piperazine

N-(2-aminoethyl)piperazine

Triethylenetetramine

Tetraethylenepentamine

Pentaethylenehexamine

Diaminoethylpiperazine

Piperazinoethylethylenediamine

4-Aminoethyltriethylenetetramine

Tetraethylenepentamine

Aminoethylpiperazinoethylethylenediamine

Piperazinoethyldiethylenetriamine

Another class of hardeners, but which can also be used as extender ofthe epoxy resin, are the higher molecular weightpoly(oxyalkylene)polyamines such as those of the following formulas:##STR6## where y is 2-40 ##STR7## where a+c is about 2.5 and b is 8-45.##STR8## where x, y and z range from about 2-40 ##STR9## where m+t isabout 82-86.

The hardener may be a monoamine such as aniline, paraaminophenol, andalkylated versions of them.

A further class of desirable hardeners are the reaction products ofdialkylamines, such as dimethylamine, diethylamine, methylethylamine,di-n-propylamine, and the like, with a variety of mono and diisocyanatesto form mono and diureas. Any of the polyisocyanates listed below my beso reacted for use as a hardener. Specific illustration of usefulhardeners are those encompassed by the following formulas anddescriptions: ##STR10## where R is a monovalent organic group; R' isalkyl, halo, alkoxy, and the like; R" is methylene, isopropylidene,ethylidene, or a covalent bond; and a is 0-4.

Preferred urea hardeners are those that are the reaction products ofdimethylamine with mixtures of 80% 2,4-tolylene diisocyanate and 20%2,6-tolylene diisocyanate, polymeric isocyanate,p-chlorophenylisocyanate, 3,4-dichlorophenylisocyanate orphenylisocyanate.

The amount of the hardener employed is usually stoichiometricallyequivalent on the basis of one amine group per epoxy group in the resin.If the epoxide is a triepoxide and the hardener is a diamine, then themolar ratio of hardener to epoxide would typically be about 2.5/3 or0.83. A typical formulation would have a weight ratio of epoxy resin tohardener of about 3/2 to about 4/1. Where any of the hardeners serveprimarily as extenders of the epoxide resin, then the amount of thehardener in the typical case will be less than that generally employedfor hardening the epoxide. Mixtures the above hardeners and with otherhardeners are within the contemplation of this invention.

Other reactive resin systems include the various thermosetting orpthermosettable resins include the bismaleimide (BMI), phenolic,polyester (especially the unsaturated polyester resins typically used inSMC production), PMR-15 polyimide and acetylene terminated resins arealso suitable.

The urethane systems represent a typical non-engineering polymer forapplications demanding less performance than, for example, the epoxyresin systems. They typically comprise the reaction of a polyisocyanate,a polyol, alone or with another active hydrogen compound, typically inthe presence of a catalyst, such as an amine catalyst. However, in thepractice of this invention, the polyurethane is a mixture of a blockedpolyisocyanate, such as the reaction product of a mono-ol or monohydroxyphenolic compound with a polyisocyanate that is an organicpolyisocyanate. This includes an organic compounds that contain at leasttwo isocyanato groups and include the hydrocarbon diisocyanates (e.g.,the alkylene diisocyanates and the arylene diisocyanates), as well asknown triisocyanates and polymethylene poly(phenylene isocyanates).

The blocked isocyanates are compounds of the formula: ##STR11## where Ris a monovalent organic group; R' is alkyl, halo, alkoxy, and the like;and a is 0-4.

Illustrative polyisocyanates for use in making the blocked isocyanatesare:

    __________________________________________________________________________    2,4'-diisocyanatotoluene                                                                          2,6-diisocyanatotoluene                                   methylene bis(4-cyclohexyl isocyanate)                                                            1,2-diisocyanatoethane                                    1,3-diisocyanatopropane                                                                           1,2-diisocyanatopropane                                   1,4-diisocyanatobutane                                                                            1,5-diisocyanatopentane                                   1,6-diisocyanatohexane                                                                            bis(3-isocyanatopropyl)ether                              bis(3-isocyanatopropyl) sulfide                                                                   1,7-diisocyanatoheptane                                   1,5-diisocyanato-2,2-dimethylpentane                                                              1,6-diisocyanato-3-methoxyhexane                          1,8-diisocyanatooctane                                                                            1,5-diisocyanato-2,2,4-trimethypentane                    1,9-diisocyanatononane                                                                            1,10-disocyanatopropyl)ether                                                  of 1,4-butylene glycol                                    1,11-diisocyanatoundecane                                                                         1,12-diisocyanatododecane                                                     bis(isocyanatohexyl) sulfide                              1,4-diisocyanatobenzene                                                                           2,4-diisocyanatotolylene                                  2,6-diisocyanatotolylene                                                                          1,3-diisocyanato-o-xylene                                 1,3-diisocyanato-m-xylene                                                                         1,3-diisocyanato-p-xylene                                 2,4-diisocyanato-1-chlorobenzene                                                                  2,4-diisocyanato-1-nitrobenzene                           2,5-diisocyanato-1-nitrobenzene                                                                   4,4-diphenylmethylene diisocyanate                        3,3-diphenyl-methylene diisocyanate                                                               polymethylene poly                                                            (phenyleneisocyanates)                                    isophorone diisocyanate                                                                           and mixtures thereof.                                     __________________________________________________________________________

The preferred polyisocyanates are mixture of 80% 2,4-tolylenediisocyanate and 20% 2,6-tolylene diisocyanate and polymeric isocyanate.The blocked isocyanates comprise any of the above polyisocyanatesreacted with a monofunctional hydroxy containing compound. The resultantblocked polyisocyanate is unreactive towards hydroxyl compounds at roomtemperature pbut, at elevated temperatures, will function as anisocyanate to crosslink the hydroxyl compounds to form the thermosetresin. For example, an adduct of tolylene diisocyanate andtrimethylolpropane is first prepared in solution, followed by theaddition of phenol to block the remaining isocyanate groups.Illustrative of such a blocked polyisocyanate is a phenol blockedtoluene diisocyanate in cellosolve acetate sold by Mobay Chemical Co.,as Mondur S. Such blocked isocyanates, when mixed with the polyols,provide a thermosetting polyurethane matrix resin that can form a tackythin in situ-expandable film that is storable and curable on demand, inaccordance with the invention.

The polyols used in forming the polyurethane may be an organic diol,triol, tetraol, pentaol, and the like. Illustrative are the followingcompounds: ethylene glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, 1,2-propylene glycol, di-1,2-propylene glycol,tri-1,2-propylene glycol, tetra-1,2-propylene glycol, 1,4-butanediol,1,3-propanediol, and the like; or formed from by the alkoxylation of astarter polyol, such as the class of polyols characterized; or formedfrom reaction of the above diols, triols, etc., with caprolactone. Theresulting ester polyols ("Tone's") are widely used in reactions withisocyanate. Desirable alkoxylated polyols are alkylene oxide adducts ofa hydroxylated alcohols of the formula:

    A(OH).sub.>1

and preferably a "starter" diol, triol, tetraol and higher hydroxylatedalcohols, of the formula:

    A(OH).sub.≧2-6

wherein A is a polyvalent organic moiety, the free valence of which is2-6 or greater, or an average value equal thereto, as the case may be.

Illustrative of suitable compounds embraced by the "starter"A(OH).sub.≧2-6 alcohol are the following: ethylene glycol, diethyleneglycol, 1,2-propylene glycol, polyethylene glycol, polypropylene glycol,glycerine, pentaerythritol, sorbitol, diether of sorbitol, mannitol,diether of mannitol, arabitol, diether or arabitol, sucrose, mixturesthereof, and the like.

The starter A(OH).sub.≧2-6 is first reacted with 1,2-alkylene oxide inan amount and under conditions sufficient to convert its hydroxyl groupsto hydroxyalkyl groups. The amount of 1,2-alkylene oxide reacted issufficient to achieve the ultimato molecular weight of the alkoxylatedpolyol adduct. The molecular weight of the alkoxylated polyol adductshould be relatively high, preferably above about 4000 (number average)and, more preferably, above about 5000. The minimum molecular weight ofthe alkoxylated polyol adduct may be about 2000. The preferred1,2-alkylene oxides are lower 1,2-alkylene oxides, such as ethyleneoxide, 1,2-propylene oxide, 1,2-butylene oxide, and the like. Theresulting polyol may be hydroxyethyl capped by reaction with1,2-ethylene oxide to provide assurance of primary hydroxyl content inthe polyol especially if the alkoxylated polyol adduct is subsequentlycoupled, not polymerized, with an organic polyisocyanate. Suchalkoxylation reactions, with consequent adduct formation, is well knownin the art. Adduct reactions may be base or acid catalyzed, with basecatalyzation preferred.

The organic polyol may be a polyester polyol, such as a polyester of adicarboxylic acid, acid halide or anhydride and a polyol, such as thosecharacterized above. In this case, it is desirable to allow the polymerto be hydroxyl terminated, and conventional procedures in the art areuseful for this purpose. A polyol is also employed to produce thepolyester. Such polyols include ethylene glycol, diethylene glycol,propylene glycol, dipropylene glycol, butylene glycols, neopentylglycol, glycerol and 1,1,1-trimethylolpropane.

Polyester resins usable as the thermosetting matrix resin, are typicallyreaction products of a dicarboxylic acid, acid halide or anhydride, witha polyhydric alcohol. The dicarboxylic acids or anhydrides that areemployed to produce the polyester, either singly or in combination,include those that contain olefinic unsaturation, preferably wherein theolefinic unsaturation is alpha, beta- to at least one of the carboxylicacid groups, saturated paliphatic, heteroaliphatic and aromaticpolycarboxylic acids, and the like. Such acids include maleic acid oranhydride, fumaric acid, methyl maleic acid, and itaconic acid (maleicacid or anhydride and fumaric acid are the most widely usedcommercially), saturated and/or aromatic dicarboxylic acids oranhydrides such as phthalic acid or anhydride, terephthalic acid,hexahydrophthalic acid or anhydride, adipic acid, isophthalic acid, and"dimer" acid (i.e., dimerized fatty acids). They may be cured byproviding a polymerization initiator and low viscosity crosslinkingmonomers in the formulation. Where the resin is a unsaturated polyesteror vinyl ester, it is preferred that the monomers contain ethylenicunsaturation such that the monomer is copolymerizable with the polyesterand terminally unsaturated vinyl ester resins. Useful monomers includemonostyrene, alkyl acrylates and methacrylates such as C₁₋₁₂ alkylacrylates and methacrylates, substituted styrenes such as α-methylstyrene, α-chlorostyrene, 4-methylstyrene, and the like, divinylbenzene,acrylonitrile, methacrylonitrile, and the like. Styrene is the preferredmonomer in commercial practice today, although others can be used.Suitable polymerization initiators include t-butyl hydroperoxide,t-butyl perbenzoate, benzoyl peroxide, cureerie hydroperoxide, methylethyl ketone peroxide, and others known to the art. The polymerizationinitiator is employed in a catalytically effective amount, such as fromabout 0.3 to about 2 to 3 weight percent, based on the weight ofpolyester and the crosslinking monomer.

When desired, a thickening agent can also be employed in the polyesterthermosetting compositions. Such materials are known in the art, andinclude the oxides and hydroxides of the metals of Group I, II and IIIof the Periodic Table. Illustrative examples of thickening agentsinclude magnesium oxide, calcium oxide, calcium hydroxide, zinc oxide,barium oxide, magnesium hydroxide and the like, including mixtures ofthe same. Thickening agents are normally employed in proportions of fromabout 0.1 to about 6 weight percent, based upon weight of the polyesterresin and crosslinking monomer.

Particularly desirable materials for rendering the thermosetting resinnon-pourable are thixotropic agents and/or elastomer-type polymers thatprovide discrete elastomer phases (second phases) in the thermosettingresin matrix. Certain of these material may reduce, to some finitedegree, the crosslinking density of the thermoset resin (C-stage). Manyof these materials introduce very favorable properties to the resultingthermoset resin. For example, a partially desirable material for thispurpose, is an elastomeric polymer containing soft and hard segments,the hard segments acting like or forming on processing, crosslinking ofthe elastomeric type. Some of these elastomeric types contain functionalend groups which allow it to couple with complementary functionalmonomers or polymers to form the desired elastomer in situ of thethermosetting resin and render it non-pourable and tacky, whiletoughening the cured resin. As a class, these elastomeric polymers actor are crosslinked yet are thermoprocessable, which when discretelyprovided in the matrix resin renders the resin non-pourable and tacky,and also toughens it.

One class of suitable elastomer-type thermosplastic ABS(acrylonitrile-1,4-butadiene-styrene) block copolymers that aretypically used as modifiers of other resin systems. They arecharacterized as having a wide range of properties though the preferredsystems of the invention utilize copolymers that are high rubber typesthat, when compared to other copolymers of this type, have a relativelylow tensile strength, low tensile modulus, higher impact resistance, lowhardness and heat deflection temperature. Another elastomer that isfound desirable are the carboxyl and amine terminated liquid butadieneacrylonitrile copolymers. Such copolymers may contain pendant carboxylgroups in the interior of the polymer structure through the inclusion ofmethacrylic or acrylic acid in the polymerization or through thehydrolysis of some of the pendant nitrile units. Such polymers reactwith the epoxy resin and as a result, the epoxy forms the hard segmentgenerating the elastomer properties.

Another class of block thermoplastic elastomers are Kraton™, availablefrom Shell Chemical Company. These thermoplastic rubber polymers possessusable thermoplastic properties. They can be softened and they flowunder heat and pressure. They then recover their structures on cooling.The chemical make-up are of three discrete blocks of the linear or A-B-Atype. They are available as styrene-butadiene-styrene (S-B-S) blockcopolymers, styrene-isoprene-styrene (S-B-S) block copolymers andstyrene-ethylene/butylene-styrene (S-EB-S) block copolymers. They arecharacterized by styrene polymer endblocks and an elastomeric midblock.After processing, the polystyrene endblocks physically crosslink,locking the rubber network in place. This physical crosslinking isreversible on heating.

Another series of the Kraton™ thermoplastic rubbers are the diblockpolymers in which one block is a hard thermoplastic and the other is asaturated soft elastomer. Illustrative of this series is Kraton™ G 1701,a diblock polymer of a hard polystyrene block and a saturated, softppoly(ethylene-propylene) block.

Other rubbers or elastomers include: (a) homopolymers or copolymers ofconjugated dienes having a weight average molecular weight of 30,000 to400,000 or higher as described in U.S. Pat. No. 4,020,036, in which theconjugated dienes contain from 4-12 carbon atoms per molecule such as1,3-butadiene, isoprene, and the like; (b) epihalohydrin homopolymers, acopolymer of two or more epihalohydrin monomer, or a copolymer of anepihalohydrin monomer(s) with an oxide monomer(s) having a numberaverage molecular weight (M_(n)) which varies from about 800 to about50,000, as described in U.S. Pat. No. 4,101,604; (c) chloroprenepolymers including homopolymers of chloroprene and copolymers ofchloroprene with sulfur and/or with at least one copolymerizable organicmonomer wherein chloroprene constitutes at least 50 weight percent ofthe organic monomer make-up of the copolymer as described in U.S. Pat.No. 4,161,471; (d) hydrocarbon polymers including ethylene/propylenedipolymers and copolymers of ethylene/propylene and at least onenonconjugated diene, such as ethylene/propylene/hexadiene/norbornadiene,as described in U.S. Pat. No. 4,161,471; (e) conjugated diene butylelastomers, such as copolymers consisting of from 85 to 99.5% by weightof a C₄ -C₇ isolefin combined with 15 to 0.5% by weight of a conjugatedmulti-olefin having 4 to 14 carbon aroma, copolymers of isobutylene andisoprene where a major portion of the isoprene units combined thereinhave conjugated diene unsaturation as described in U.S. Pat. No.4,160,759.

Specific illustrations of suitable elastomeric polymers are thefollowing:

1. Hycar™ CTBN liquid reactive rubbers, carboxyl terminatedbutadiene-acrylonitrile copolymers sold by B. F. Goodrich.

2. Hycar™ CTBNX, similar to CTBN except that they contain internalpendant carboxyl groups, also supplied by B. F. Goodrich.

3. Hycar™ ATBN, amine terminated butadiene-acrylonitrile copolymers soldby B. F. Goodrich.

4. K 1102-28:72 styrene:butadiene linear SBS polymer, available fromShell Chemical Company under the registered trademark "Kraton" 1102.

5. KDX 1118-30:70 styrene:butadiene copolymer containing 20% SBStriblock and 80% SB diblock, available from Shell Chemical Company underthe registered trademark "Kraton" DX 1118.

6. KG 1657-14:86 styrene:ethylene-butylene:styrene copolymer availablefrom Shell Chemical Company under the registered trademark "Kraton"G1657.

7. S 840 A-Stereospecific 43:57 styrene-butadiene SB rubber availablefrom Firestone Synthetic Rubber & Latex Company under the registeredtrademark "Stereon" 840A.

8. SBR 1006-random 23.5:76.5 styrene:butadiene SB block copolymer rubberavailable from Goodrich Chemical Company under the registered trademark"Ameripol" 1006.

9. SBR 1502-Random 23.5:77.5 styrene:butadiene rubber available fromHules Mexicanos, or from Goodrich Rubber Company as "Ameripol" 1502.

10. Cycolac™ Blendex modifier resins (e.g., 305, 310, 336 and 405)-ABSpolymers sold by Borg-Warner Chemicals, Inc. Different varieties areavailable and their suitability depends on the properties sought.

Illustrative of thixotropic agents that can render a thermosettableresin non-pourable are high surface area fumed silicas and organosilylblocked fumed silicas, and the like.

The thermoplastic polymer used in forming the in situ-expandablethermoplastic particles are readily prepared from a wide variety ofmaterials. A number of patents refer to their manufacture. For example,U.S. Pat. No. 3,615,972 describes their preparation by polymerizing themonomer of an aqueous dispersion of (1) organic monomeric materialssuitable for polymerization to a thermoplastic resinous material havingthe desired physical properties, (2) a liquid blowing or raising agentwhich exerts a little solvent action on the resulting polymer, and in aquantity in excess of that which is soluble in the polymer, and (3) adispersion stabilizing material which is utilized to maintain thedispersion. The resulting solid spherical particles have a quantity ofthe liquid-blowing agent encapsulated in them as a distinct and separatephase.

The thermoplastic polymers are formed by the polymerization of one ormore of a variety of different types of alkenyl monomers, such as thoseof the formula: ##STR12## p to form homopolymers or copolymers, such asrandom or ordered (including block) copolymers. In the above formula,R^(o) may be alkyl, such as methyl, ethyl and the like, or halogen, suchas chlorine, fluorine, bromine or iodine, and X may be an aromaticcontaining moiety bonded via an aromatic carbon atom, a carbonyl oxyester moiety, halogen, cyano, oxycarbonyl ester, carboxyl, and the like.Illustrative of these monomers are those in which X is aromaticcontaining, such as styrene, o-methylstyrene, m-methylstyrene,p-methylstyrene, ethylstyrene, ar-vinyl-xylene, ar-chlorostyrene,ar-bromostyrene, vinylbenzylchloride, p-tert.-butylstyrene, and thelike. Also illustrative of these monomers are those in which X is acarbonyl oxy ester moiety to form acrylate monomers alone or incombination with the alkenyl aromatic monomers may also be utilized.Such acrylate-type monomers include methyl methacrylate, ethyl acrylate,propyl acrylate, butyl acrylate, butyl methacrylate, propylmethacrylate, butyl methacrylate, lauryl acrylate, 2-ethyl hexylacrylate, ethyl methacrylate, and the like. X and R^(o) may be ahalogen, such as chlorine, fluorine, bromine and iodine, thereby toencompass the formation of copolymers of vinyl chloride and vinylidenechloride, acrylonitrile with vinyl chloride, vinyl bromide, and similarhalogenated vinyl compounds. X may be a cyano group and this includespolymers of acrylonitrile and methacrylonitrile. When X is anoxycarbonyl esters, such as the vinyl esters, such as, vinyl acetate,vinyl butyrate, vinyl stearate, vinyl laurate, vinyl myristate, vinylpropionate, and the like, are suitable polymeric components. One mayalso employ for specific purposes ethylenically unsaturatedcopolymerizable acids such as acrylic acid, methacrylic acid, itaconicacid, citraconic acid, maleic acid, fumaric acid, vinylbenzoic acid, andthe like.

The thermoplastic polymers may also include copolymers (of the random orordered varieties, especially blocked copolymers) of the monomersdescribed above with a variety of hydrocarbon monomers, such aspropylene, butene, and one or more dienes, such as:

straight chain acyclic dienes such as: 1,4-hexadiene, 1,6-octadiene, andthe like;

branched chain acyclic dienes such as: 5-methyl-1,4-hexadiene,3,7-dimethyl-1,6-octadiene, 3,7-dimethyl-1,7-octadiene and the mixedisomers of dihydro-myrcene, dihydroocinene, and the like;

single ring alicyclic dienes such as: 1,4-cyclohexadiene,1,5-cyclooctadiene, 1,5-cyclododecadiene, and the like;

multi-ring alicyclic fused and bridged ring dienes such as:tetrahydroindene, methyltetrahydroindene, dicyclopentadiene,bicyclo-(2,2,1)-hepta-2,5-diene, alkenyl, alkylidene, cycloalkenyl andcycloalkylidene norbornenes such as 5-methylene-2-norbornene (MNB),5-ethylidene-2-norbornene (ENB), 5-propyl-2-norbornene,5-isopropylidene-2-norbornene, 5-(4-cyclopentenyl)-2-norbornene,5-cyclohexylidene-2-norbornene, and the like.

The thermoplastic polymer used in forming the in situ-expandablethermoplastic particles may also be made from condensation typepolymers, such as nylon-6,6; nylon-6; nylon-4,6; polyester frompolyethylene terephthalate; Kevlar™ polyaramide; polycarbonates (viz.,poly (2,2-bis(1.4-oxyphenyl) propane carbonate)); polyarylates (viz.,poly (2,2-bis(1.4-oxyphenyl) propane terephthalate); polyimides;polyetherimides, such as Ultem™³ ; polysulfones (see U.S. Pat. Nos.4,175,175 and 4,108,837), such as Udel™ and Radel™ A-400⁴ ; thepolyethersulfones (see U.S. Pat. Nos. 4,008,203, 4,175,175 and4,108,837), such as Victrex™ PES⁵ ; polyarylsulfones;polyarylamideimides, such as Torlon™⁶ ; and the like.

A wide variety of blowing or raising agents may be incorporated withinthe polymerization system. They can be volatile fluid-forming agentssuch as aliphatic hydrocarbons including ethane, ethylene, propane,propylene, butene, isobutylene, neopentane, acetylene, hexane, heptane,or mixtures of one or more such aliphatic hydrocarbons having amolecular weight of a least 26 and a boiling point below the range ofthe softening point of the resinous material when saturated with theparticular blowing agent utilized.

Other suitable fluid-forming agents are the chlorofluorocarbons such asthose described in U.S. Pat. No. 3,615,972 (column 4, lines 21-30) andtetraalkyl silanes such pas tetramethyl silane, trimethylethyl silane,trimethylisopropyl silane and trimethyl-n-propyl silane. As pointed outin this patent, the boiling point of such foaming agents at atmosphericpressure should be about the same temperature range or lower than thesoftening point of the resinous material employed.

As pointed out in U.S. Pat. No. 4,397,799, patented Aug. 9, 1983, theparticle size of the unexpanded particles, as well as the expandedmicrospheres can vary widely. Particle sizes for the unexpandedparticles can range, for example, from about 1 μm to about 1 mm,preferably from about 2 μm to about 0.5 mm. One version of insitu-expandable particles are sold under the name Expancel®, by NobelIndustries Sweden, Sundsvall, Sweden (U.S. address: Marrietta, Ga.30062). They range in unexpanded particle size from about 5 μm to about50 μm. The particle diameters expand 2 to 5 times.

Blowing agents such as the Freons®, such as trichlorofluoromethane,hydrocarbons such as n-pentane, i-pentane, neo-pentane, butane,i-butane, azodicarbonamide are commonly suggested blowing agents foundin these types of in situ-expandable particles. Typically, theunexpanded particles contain from about 3 to about 40 weight % blowingagent.

Preferably, the particles used have a mixed particle size of wide spreadto achieve the best packing, on expansion, in the syntactic molded foam.A particularly preferred in situ-expandable particle is Expancel® 091DU, which is believed to be a terpolymer of vinylidene chloride,acrylonitrile and methacrylonitrile containing 10-18 weight %isopentane, and possesses the following properties: average unexpandedparticle size of about 12 μm with a spread of about 5-50 μm, truedensity (expanded in water at 100° C., kg/m), <20; TMA-T(start) ° C.,125-130; T(max) ° C., ˜183; TMA-density,kg/m³, <17.

As noted above, the thin film may contain fibers. Such fibers providetoughness properties to the molded syntactic foam product. Fibers thatmay be used in the practice of the invention may be any organic fiberthat has a melting temperature (T_(m)) greater than the cure temperatureof the matrix resin in making the expanded molded syntactic foam. Alsousable in the practice of this invention are fibrous type of structures,having a length greater than diameter, that are made of amorphouspolymers. For example, certain polysulfone fibers having a high T_(g)may be employed. In such a case, the polymer's T_(g) should be greaterthan the cure temperature of the matrix resin. Suitable fibers may bemade from any of the performance and engineering plastics. For example,the fibers may be made from nylon-6,6; nylon-6; nylon-4,6; polyesterfrom polyethylene terephthaiate; polypropylene; Kevlar® polyaramide;polycarbonates (viz., poly (2,2-bis(1.4-oxyphenyl) propane carbonate));polyarylates (viz., poly (2,2-bis(1.4-oxyphenyl) propane terephthalate);polysulfides (see U.S. Pat. No. 3,862,095); polyimides; polyetherimides,such as Ultem®⁷ ; polyetheretherketones, such as Victrex® PEEK⁸ andpolyetherketone or polyetherketoneketone, such Stilan® PEK or PEKK⁹ ;polysulfones (see U.S. Pat. Nos. 4,175,175 and 4,108,837), such as Udel®and Radel® A-400¹⁰ ; the polyethersulfones (see U.S. Pat. Nos.4,008,203, 4,175,175 and 4,108,837), such as Victrex® PES¹¹ ;polyarylsulfones; polyarylamideimides, such as Torlon®¹² ; and the like.

The preferred fibers are those made from the engineering plastics, suchas the polyarylethers which include the polyetherimides, thepolyetheretherketones, the polyetherketones, the polyetherketoneketone,the polysulfones, the polyethersulfones, the polyarylsulfones, thepolyarylamideimides, and the like. Particularly preferred fibers arethose made from polyetheretherketones, polyetherimides, polyarylamides,polyarylamideimides, polysulfones, polyethersulfones and polycarbonates.

The fibers are typically in the form of short cut fibers, i.e., staplefibers, ranging from about 2.5 millimeters to about 13 millimeters.Longer fibers may be used and when they are used, they typicallyconcentrate at or near the syntactic foam's surfaces, as a result ofmigration during expansion. The diameter of the fibers may rangeconsiderably. Preferably, the fiber diameter ranges from about 20 μm toabout 70 μm, preferably from about 30 μm to about 60 μm.

A typical resin formulation comprises the following:

    ______________________________________                                                                       Preferred                                                           Percent   Percent                                        Formulation          By Weight By Weight                                      ______________________________________                                        Bisphenol A epoxy resin                                                                            40 to 80  60 to 75                                       * * *                                                                         A preferred resin comprises a mix-                                            ture of (a) a solid resin and a liquid                                        resin that yield a soft, non-pourable,                                        tacky, resin; or (b) a mixture of liq-                                        uid redw that yield a soft, non-                                              pourable, tacky resin; or (c) a mix-                                          ture of a liquid Bisphenol A epoxy                                            resin and low molecular weight                                                novolak epoxy resin.                                                          An elastomer toughening agent.                                                                      0 to 12  3 to 9                                         * * *                                                                         A desirable roughening agent may be                                           a carboxylated butadiene acrylonit-                                           rile copolymer elastomer, an ABS                                              block copolymer elastomer, and SBS                                            block copolymer elastomer.                                                    Hydroxyl extender for the epoxy re-                                                                 0 to 20   4 to 12                                       sin(s).                                                                       * * *                                                                         The preferred extender is biophenol                                           A.                                                                            Amine curing agent.   4 to 12   5 to 10                                       * * *                                                                         Preferred amine curing agents ill-                                            clude aliphatic amities, alkylene                                             oxide amities, aromatic amities and                                           aromatic ureas.                                                               Diluent.              0 to 2   0                                              * * *                                                                         A variety of conventional ether,                                              ketone, acetate, and the like diluent                                         may be added to facilitate com-                                               patibility. They are typically re-                                            moved by evaporation once the film                                            is formed.                                                                    Thermoplastic fibers  0 to 20   6 to 14                                       ______________________________________                                    

These resin formulations are made by conventional mixing of thecomponents in standard mixing equipment for viscous compositions. Goodresults have been obtained using a Ross® Double Planetary Mixer,provided with vacuum construction and jacketing to control temperatureand deaerate the mixture. Mixing is typically effected by blending theresin, unexpanded particles, elastomer components, extenders, diluents,curing agent and fibers (these being added last), and vacuum pumping toremove entrained air. The temperature chosen is variable depending onthe viscosity of the formulation. It may be desirable to separately mixthe resin and the curing agent. In such a case, the formulation may bedivided up to mix the resin with some portion of the formulation toeffect a well dispersed condition and do the same with the curing agent,and then combine the well dispersed mixes with the fiber component andthe unexpanded particles, so as to mix them all under conditionsavoiding premature reaction. Such procedures are well within the skillof the art.

Calenaring of the resin formulation into the thin films of the inventionare illustrated in the drawings. As shown in FIG. 1, which is anisometric-like schematic illustration of a calendaring line 1 forcalendaring a nonreinforced film, thermosetting matrix resin formulation(containing punexpanded, in situ-expandable particles) feed 3 is fed tonip rolls 5. Nip rolls 5 are calendar rolls spaced apart to the desiredthickness of the film 7. It is desirable in the practice of theinvention to avoid drawing action of film 7 after extrusion formation byrolls 5. Rolls 5 may vary in width, wider rolls generating morethroughput and narrower rolls providing more control over film thicknessfrom edge to edge. Because this invention is concerned with films ofessentially uniform thickness from edge to edge, and front to back, itis desirable to use calendar rolls that are less than about 36 incheswide. A convenient width is about 12 to about 18 inches. Manufacture offilms meeting the specifications of this invention are easier at thosewidths. Because the viscosity of feed 3 is not excessive, one may lookat the calendaring operation as a filming operation, akin to rollercoating. The distance between rolls 5 is maintained by a force balance(not shown) between the hydraulic pressure pushing on the roll and theoff-setting matrix fluid pressure acting in the opposite direction tothe roll.

Once film 7 is formed, it is frequently desirable to reduce the matrixresin viscosity in the film. Temperature reduction of film 7 reducesviscosity which reduces flow within the film and thus helps to preserveits dimensions. This may be accomplished by passing film 7 over one ormore chilled rollers 9, 13 and 15. If used as chilled rollers, they aretypically internally cooled via internal jacketing, to temperatures fromabout 0° C. to about 25° C., sufficiently low enough to prevent anysagging or flow of the resin matrix. The chill rollers, by cooling thefilm, increase the resin's elastic modulus so that resin flow isdecreased and film dimensional stability is maintained. In theconfiguration of FIG. 1, roller 9 may be utilized as a chilled roller, aguide roller for alignment purposes and/or a take-up roller, as desired.For handling convenience, release paper or plastic (viz., polyethylenefilm) layers 12 and 14 may be applied to the outside surfaces of film 7,from their corresponding core rolls, under or over rolls 13 and 15, asmay be the case, to form a sandwiched construction. The so protectedfilm 19, as a sandwiched construction, is rolled up onto core 17.

FIG. 2 shows another calendaring line, 21, which comprises feed 23containing expandable particles, calendar rolls 25, film 27, rollers 29,33 and 35, core and film 32, corresponding to feed 3, calendar rolls 5,film 7, rollers 9, 13 and 15, core and film 12, respectively, of FIG. 1.What is different in FIG. 2 is the use of a scrim layer 41, comprised ofan open woven, nonwoven or knitted scrim construction, that is guided byroller 43 to roller 35 to be pressed into contact with film 27 by way ofpassage under roller 35. This sandwiched construction is collected asroll 39 on core 37.

FIG. 3 offers another calendaring line, 51, comprising feed 53, calendarrolls 55, film 57, rollers 59, 63 and 65, core and film 62,corresponding to feed 3, calendar rolls 5, film 7, rollers 9, 13 and 15,core and film 12, respectively, of FIG. 1 and scrim layer 71, comprisedof an open woven, nonwoven or knitted scrim construction, that is guidedby roller 43 to roller 35 to be pressed into contact with film 27 by wayof passage under roller 35. In line 51, the difference is the inclusionof an additional chiller roller 58 to control the viscosity of film 57,and a second film line, in order to form a two-layer film compositestructure. The second film line comprises matrix feed 75, withunexpanded particles, that is formed into film 79 by calendar rolls 77,passed over chilled roller 82, guided and further cooled by guide andchilled roller 81. It is then merged into contact with scrim layer 71,film layer 61, and release layer 62 at roller 65, and the composite thenpassed over guide roller 83 to be taken up as composite film 69 on core67.

The invention also contemplates the inclusion of one or more layers of anonwoven fabric provided with a resin binder that is co-curable with thematrix resin. These added layers serve to enhance the impact andbuckling resistance of the composite structure. The non-woven layer istypically provided as an outside layer, such as a substitute for releaselayer 62, to provide a support surface for the thin

The nonwoven structures may be formed from unspun or spun staple fibershaving a length of from about 1/4 inch to about 3 inches by garnettingand crosslaying, airlaying on a rotating screen or on an endless tenterarrangement according to the procedure of U.S. Pat. No. 3,538,564,utilizing the apparatus of U.S. Pat. Nos. 3,345,231 and 3,345,232. Thenonwoven structures may be resin impregnated by spraying thethermosetting resin as a solvent solution into the batting or scrim-likestructures. Preferably, the nonwoven is first bonded with a low costthermoplastic from a latex or water dispersion or with starch from anaqueous solution, dried to fix the fibers in the nonwoven structure, andthen pthe nonwoven structure is impregnated with the thermosettingresin. The nonwoven can be supported by a scrim layer in much the samemanner that the thin film is supported by one or more scrim layers 71.

FIG. 4 is a simpler and preferred method for making a scrim-supportedcomposite. In line 101, scrim layer 103 is fed centered of the spacebetween calendar rolls 107 and the thermosetting resin matrix feed 105with the unexpanded particles is uniformly applied to both of the rollsso that there is an essentially equivalent amount on both sides of scrimfilm 103. At the same time, release layer 106, supplied from its core,is passed over one of the rolls 107 to form an outside release surface.This composite is passed over chilled and guide roller 111, to formsandwiched film 113 containing matrix resin with a thin inner scrimlayer. The sandwiched film is passed over guide and chilled rollers 115and 117 to be collected as composite film 121 on core 119.

FIG. 5 shows another way of forming a scrim composited film. In system130, the scrim layer 103 is fed to the calendar rolls 107 with thematrix resin feed shown to contain particles of in situ-expandablethermoplastic. In this system, outside release layers 131 and 133 arepassed over calendar rolls 107 to insure that the film is formed betweenthem. The resulting film can be cooled and collected as noted above.

In FIG. 6, system 135 is the same as system 130 of FIG. 5, except thatthe resin matrix 137 also contains short staple fiber with the thermalcharacteristics noted above. In the preferred case, the fiber length isabout 1/4 inch. The mass is film-formed and the construction has thecharacteristics shown in FIG. 7. In FIG. 7, the film 139 comprisesstaple fibers 141 well dispersed in the non-pourable film. Also presentin the film, but not illustrated, are the unexpanded, in situ-expandablethermoplastic particles. The fibers are caused to be oriented in thefilm more in the direction of flow of the matrix resin, consequently,more fibers will be found to be oriented parallel to the film'ssurfaces.

Molding of the films of the invention to produce molded syntactic foamsis simple and straightforward. For example, as shown in FIG. 8, a mold143, shown as having an open end, is a split mold of the configurationshown. In this case, the mold is a slice of a cylinder. The mold definesthe shape of the resulting syntactic foam. Mold 143 comprises arcuatetop and bottom walls 145 and 149 and arcuate front and back walls 150and 148, and sidewalls 146. The unexpanded film, as one or more layers147 are cut to the shape and size of surface 149, and then laid ontothat surface. The mold is then closed and heat is applied to the mold.The temperature to which the mold is heated is dependent upon severalconsiderations, such as the temperature at which crosslinking of thethermosetting matrix resin is initiated, the melting and/or second ordertransition temperature of the fiber, if present, the desired syntacticfoam density if the blowing agent used in the particle overlaps thecrosslinking temperature of the resin, and the like considerations. Withthe use of epoxy resins, the cure temperature is typically at 350° F.(177° C.) or 250° F. (121° C.).

In the above noted processes, the release layers may be substituted forby other materials such as syntactic foams comprising rigidmicroballoons in a resin matrix comprises microballoons (microspheres)embedded in the uncured or partially cured matrix resin. In this case,the matrix resin may be any of the resins described above with respectto the film of the invention. The most common of the microballoons aremade of glass, but quartz, phenolic, carbon, thermoplastic and metalcoated microballoons are usable. A Syncore® is suitable for thispurpose.

The microballoons in those syntactic foam films are synthetic hollowmicrospheres that comprise individual round spheres or bubbles havingdiameters which range from about 1 to about 500 microns, preferablyabout 1 to about 200 microns, with wall thicknesses of about 0.1 toabout 20 microns. They typically possess densities ranging from about0.1 to about 0.5 g./cc. The syntactic foam comprising the rigidmicroballoons in a resin matrix as a result have relatively lowdensities such as densities ranging from about 0.5 to about 0.7 g./cm.₃.Glass is the most common microballoon material in these types ofmaterials, but quartz, phenolic, carbon, thermoplastic and metal coatedmicroballoons are suitably employable.

Such syntactic foam used in composites with the films of the inventionmay have a thickness ranging from about 0.007 to about 0.125 inch. Eachsuch film would be uniform in thickness.

I claim:
 1. A tacky and drapable, non-pourable film having a uniformthickness, that contains (i) a non-pourable thermosetting matrix resinsystem and (ii) particles of a microcellular in situ-expandablethermoplastic polymer containing an expansion agent therein in whichboth (i) and (ii) are uniformly distributed throughout the film, so thatupon expansion of the thermoplastic polymer into microcells in thenon-pourable film, the resulting film is a thermoset thin film syntacticfoam the thickness of which is about 1.01 to about 4 times greater thanthe non-expanded film, which film is composited with at least one layerof another material.
 2. The thin and drapable, in situ-expandable tackyfilm of claim 1 wherein the other material strengthens the film prior toexpansion and conversion to the thermoset state.
 3. The thin anddrapable, in situ-expandable tacky film of claim 2 wherein the othermaterial is calendared to the film.
 4. The thin and drapable, insitu-expandable tacky film of claim 3 wherein the other layer ofmaterial comprises scrims, foils and plastic films.
 5. The thin anddrapable, in situ-expandable tacky film of claim 3 wherein the othermaterial sandwiches the film.
 6. The thin and drapable, insitu-expandable tacky film of claim 3 wherein the bond of the film tothe other material is effected through the tackiness of the film.
 7. Thethin and drapable, in situ-expandable tacky film of claim 3 wherein thematerial is an open scrim.
 8. The thin and drapable, in situ-expandabletacky film of claim 3 wherein the open scrim is one or more of a woven,nonwoven or knitted scrim.
 9. The thin and drapable, in situ-expandabletacky film of claim 1 wherein the film is composited with an uncuredsyntactic foam that comprises thin films of uniform thickness whichcontain rigid microballoons uniformly dispersed in a resin matrix. 10.The thin and drapable, in situ-expandable tacky film of claim 9 whereinthe resin matrix in the film and the uncured syntactic foam arecocurable.
 11. The thin and drapable, in situ-expandable tacky film ofclaim 1 wherein the thin and drapable, tacky film is composited with alayer of a prepreg.
 12. The film of claim 1 scrolled into a smalldiameter tube about which is adhered one or more prepreg layerscontaining carbon fiber reinforcement to form a composite tubecontaining a small hole in the center.